1. Field of the Invention
This invention relates to a compression coil spring used as, for example, a vehicle suspension spring or the like and a manufacturing device and manufacturing method for a coil spring.
2. Description of the Related Art
Manufacturing devices that manufacture coil springs by hot or cold forming of materials are known as coil spring manufacturing devices. A hot coiling device is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-20641 (Patent Document 1). This coiling device comprises a mandrel on which a heated material is wound and a guide mechanism that guides the material such that the material is wound at a predetermined pitch. The distal end portion of the material is chucked by a clamping mechanism attached to the mandrel. Those parts of the material (not having been wound on the mandrel yet) other than the distal end portion are in a free state without restraint. The material is spirally formed by being wound on the mandrel. The formed coil spring is subjected to heat treatment, such as quenching, tempering, etc. Compressive residual stress is produced on the surface of the spring by shot peening after the heat treatment.
In order to manufacture a coil spring with a relatively small diameter, a coiling device is used such that a material is wound cold on a mandrel. In the cold-formed coil spring, compressive residual stress is caused on the outside of a bend of the spring by spring-back, and tensile residual stress is caused on the inside of the bend of the spring. Tensile residual stress adversely affects the durability of the coil spring. If necessary, therefore, the tensile residual stress on the surface inside the bend is reduced or compressive residual stress is produced on the surface by stress relieving annealing or shot peening.
It is important for a coil spring used as a vehicle suspension spring to have reduced weight and improved durability to overcome higher-stress application. The durability of a compression coil spring, which is designed to repeat elastic deformation, is considerably influenced by stress produced during the deformation. It is known, in particular, that the lower the tensile stress on the surface of the spring, the more the durability of the spring is improved. Accordingly, such a method is adopted that the durability of the spring is improved by previously applying compressive residual stress to the surface of the spring. A typical example of this method is shot peening.
However, this conventional method requires equipment and a process for shot peening independent of coiling (plastic working). This results in an increase in product costs. It is not easy, moreover, to produce sufficient compressive residual stress on the inner surface side of the coil spring by shot peening. If possible, therefore, shot peening is expected to be omitted.
The inventors hereof proposed to control the residual stress distribution after working for a preferred level by intentionally applying loads, such as tensile and torsional loads, as well as necessary loads for original plastic working (coiling) during a cold plastic working for the spring. The special point is to confine the tensile residual stress that adversely affects the durability performance of the spring within the region from the surface of the spring to the interior of the spring.